In recent years, the increasingly widespread use of display device alternatives to the cathode ray tube (CRT) has driven the demand for large-area electronic arrays. In particular, amorphous silicon and laser re-crystallized polycrystalline silicon (poly-silicon) are used to drive liquid crystal displays commonly used in laptop computers. However, fabricating such large-area arrays is expensive. A large part of the fabrication cost of the large-area arrays arises from the expensive photolithographic process used to pattern the array. In order to avoid such photolithographic processes, direct marking techniques have been considered an alternative to photolithography.
An example of a direct marking technique used in place of photolithography involves utilizing a xerographic process to deposit a toner that acts as an etch mask. However, toner materials are hard to control and difficult to remove after deposition.
Another example of a direct marking technique involves “digital lithography” in which a droplet source including, for example, an inkjet printhead, is used to deposit a liquid mask onto a substrate in accordance with predetermined printing data. A problem with digital lithography is that inkjet printing of functional devices is susceptible to several defect creation processes during the printing operation: misdirected ejection, ejection failure, droplet/spot size variation, alignment error, etc. In most device printing applications, single defects, depending on their nature, will result in a device that will not function to specifications.
It is highly desirable to develop robust digital lithography systems that maximize yields. Currently, the method of quality control for micro electronic and optical pattern formation by digital lithography involves post-printing inspection of the pattern after the entire substrate is patterned. While post-printing inspection facilitates finding printing errors caused by a defective printhead/ejector, the location of the defective printhead/ejector may not be readily apparent when the defective printhead/ejector is one of several printheads/ejectors operating in parallel, thus making it necessary to both scrap the defective substrate and to perform a separate test to identify the defective printhead/ejector prior to resuming production. In rare instances, after finding and replacing the defective printhead/ejector, post-processing of the defective substrate may be attempted to correct printing errors. However, such corrections are performed well after deposited materials have gone through a phase change (i.e., assumed a solid form), thereby producing inferior correction results because the corrective liquid mask may not adhere well to the already-solid mask material.
What is needed is a multi-ejector digital lithography system that identifies a defective ejector immediately after its failure, and initiates immediate corrective action, thereby minimizing interruption of the printing process and producing superior corrective results. What is also needed is a method for identifying a defective ejector from a plurality of parallel ejectors, and to deactivate the defective ejector and activate an associated redundant ejector in a manner that minimizing interruption of the printing process.